A lithium ion secondary battery has been widely used as, for example, power supply for personal digital assistances because of its high energy density and high capacity. In recent years, it has been increasing its use in industry, such as in electric hybrid vehicles that require higher-capacity batteries, and studies have been carried out to increase the capacity and improve the performance. One of the approaches is to use silicon, tin, or their alloys having a high lithium intercalation capacity per unit volume as a negative electrode active material thereby to increase the discharge capacity.
The problem with lithium ion secondary batteries using such a negative electrode active material is that the active material layer can break or separate from the current collector when a binder resin widely employed in electrodes using carbon as an active material, such as polyvinylidene fluoride or a rubber resin, is used to bind the active material or bond the active material layer to the current collector because of larger changes in volume of the negative electrode active material with charge and discharge cycles. If the current collection structure of the negative electrode is so destroyed, the electron conductivity inside the negative electrode reduces, resulting in reduction of cycle characteristics. It has therefore been demanded to develop a binder resin having good toughness to provide resistance against breakage or separation that may be caused by large volumetric changes.
To meet the demand, patent literatures 1 to 4 (see below) propose using high-strength resins, such as polyimide, as a binder resin for a negative electrode active material. Patent literature 5 discloses using a polyimide having an elastic modulus of 3 GPa or more. Patent literature 6 describes a binder resin comprising a polyimide having a 3,3′,4,4′-benzophenonetetracarboxylic acid residue. However, there are no concrete proposals about a polyimide-based binder resin having high toughness which is necessary to retain the characteristics despite large volumetric changes, i.e., high elongation at break and high breaking energy, and a composition containing a solution of a precursor thereof.
Patent literature 7 describes polyimide from a polyamic acid composed of 3,3′,4,4′-biphenyltetracarboxylic acid as a tetracarboxylic acid component and 4,4′-oxydianiline and 1,3-bis(4-aminophenoxy)benzene as a diamine component. Patent literature 8 discloses a crystalline polyimide resin capable of melt molding. However, both literatures are silent on the mechanical characteristics and have no mention of use as an electrode binder resin.